In recent years, environmental problems have become an issue of great concern on a global scale. In the automotive industry, environmental protection measures have been actively promoted in manufacturing processes. In automotive manufacturing processes, reducing the amount of released volatile organic compounds (VOC) has become a particularly urgent task.
The outer panels, etc., of automobile bodies are usually coated with a multilayer coating film comprising an undercoating film, an intermediate coating film and a topcoating film to impart corrosion resistance and a beautiful appearance. Of these coats, the undercoating film is formed by applying an aqueous coating composition such as a cationic electrodeposition coating composition, etc. To reduce the amount of VOCs, the use of aqueous intermediate and top coating compositions for forming intermediate and top coating films, respectively, has been actively promoted.
An aqueous intermediate coating composition is required to form a coating film with excellent smoothness, masking slight unevenness on the surface of an undercoating film such as an electrodeposition coating film, etc. An intermediate coating film is usually formed by applying an intermediate coating composition on an undercoating film, and subsequently conducting the steps of pre-drying and curing by heating. The pre-drying step volatilizes and dries out a medium such as water, an organic solvent, etc., contained in a composition. The step of curing by heating first uses heat to melt a deposited coating composition to allow it to flow, followed by a curing reaction to obtain a cured coating film.
Japanese Unexamined Patent Publication No. 2002-294148 proposes, as an aqueous intermediate coating composition that exhibits good background hiding properties, an aqueous intermediate coating composition comprising a polyester resin containing carboxyl groups and an alkyl-etherified melamine resin whose mononuclear triazine content is 50% by weight or more, and whose degree of polymerization is 2.5 or less. However, since this aqueous intermediate coating composition does not exhibit sufficient heat flow properties for a deposited coating film, the background hiding properties are reduced, resulting in the formation of a coating film with unsatisfactory smoothness.
In order to solve the above-described problems, Japanese Unexamined Patent Publication No. 2003-105257 discloses an intermediate coating composition of a similar component composition, in which the hiding properties are improved by reducing the viscosity of the deposited coating composition, and delaying the curing initiation time to obtain better heat flow properties. However, even with this coating composition, satisfactory background hiding properties, and by extension, a smoother surface, cannot be obtained in the coating film.